Microbial community shifts reflect losses of native soil carbon with pyrogenic and fresh organic matter additions and are greatest in low-carbon soils

Soil organic carbon (SOC) plays an important role in regulating global climate change, carbon and nutrient cycling in soils, and soil moisture. Organic matter (OM) additions to soils can affect the rate at which SOC is mineralized by microbes, with potentially important effects on SOC stocks. Understanding how pyrogenic organic matter (PyOM) affects the cycling of native SOC (nSOC) and the soil microbes responsible for these effects is important for fire-affected ecosystems as well as for biochar-amended systems. We used an incubation trial with five different soils from National Ecological Observatory Network sites across the US and 13C-labelled 350 C corn stover PyOM and fresh corn stover OM to trace nSOC-derived CO2 emissions with and without PyOM and OM amendments. We used high-throughput sequencing of rRNA genes to characterize bacterial, archaeal, and fungal communities and their response to PyOM and OM. We found that the effects of amendments on nSOC-derived CO2 reflected the unamended soil C status, where relative increases in C mineralization were greatest in low-C soils. OM additions produced much greater effects on nSOC-CO2 emissions than PyOM additions. Furthermore, the magnitude of microbial community composition change mirrored the magnitude of increases in nSOC-CO2, indicating a specific subset of microbes were likely responsible for the observed changes in nSOC mineralization. However, PyOM responders differed across soils and did not necessarily reflect a common "charosphere". Overall, this study suggests that soils that already have low SOC may be particularly vulnerable to short-term increases in SOC loss with OM or PyOM additions.

土壤有机碳（Soil Organic Carbon, SOC）在调控全球气候变化、土壤碳氮循环及土壤水分方面发挥着关键作用。向土壤中添加有机物质（Organic Matter, OM）可影响微生物对SOC的矿化速率，进而对SOC储量产生潜在重要影响。明确热解有机物质（Pyrogenic Organic Matter, PyOM）如何调控原生土壤有机碳（native SOC, nSOC）的循环，以及介导该过程的土壤微生物，对于受火灾影响的生态系统及施加生物炭的土壤系统均具有重要意义。本研究针对美国境内国家生态观测站网络站点的5种不同土壤，采用13C标记的350℃玉米秸秆热解有机物质与新鲜玉米秸秆有机物质开展培养试验，以追踪施加与未施加PyOM、OM处理下源自nSOC的CO₂排放通量。本研究通过对核糖体RNA基因进行高通量测序，解析细菌、古菌与真菌群落组成及其对PyOM和OM添加的响应特征。研究结果显示，外源添加物对nSOC来源CO₂排放的影响取决于未处理土壤的碳库状态：低有机碳土壤的碳矿化相对增幅最为显著。相较于PyOM添加，OM添加对nSOC来源CO₂排放的影响更为显著。进一步分析发现，微生物群落组成的变化幅度与nSOC来源CO₂排放的增幅呈正相关，表明特定类群的微生物极有可能介导了观测到的nSOC矿化变化。不过，响应PyOM添加的微生物类群存在土壤特异性，并未形成统一的“炭圈（charosphere）”特征。综上，本研究表明，原生SOC含量较低的土壤在施加OM或PyOM后，其SOC短期流失量可能出现尤为显著的上升。